[SPARK-49000][SQL][FOLLOWUP] Improve code style and update comments

[uros-db]

What changes were proposed in this pull request?

Fix RewriteDistinctAggregates rule to deal properly with aggregation on DISTINCT literals. Physical plan for select count(distinct 1) from t:

-- count(distinct 1)
== Physical Plan ==
AdaptiveSparkPlan isFinalPlan=false
+- HashAggregate(keys=[], functions=[count(distinct 1)], output=[count(DISTINCT 1)#2L])
   +- HashAggregate(keys=[], functions=[partial_count(distinct 1)], output=[count#6L])
      +- HashAggregate(keys=[], functions=[], output=[])
         +- Exchange SinglePartition, ENSURE_REQUIREMENTS, [plan_id=20]
            +- HashAggregate(keys=[], functions=[], output=[])
               +- FileScan parquet spark_catalog.default.t[] Batched: false, DataFilters: [], Format: Parquet, Location: InMemoryFileIndex(1 paths)[file:/Users/nikola.mandic/oss-spark/spark-warehouse/org.apache.spark.s..., PartitionFilters: [], PushedFilters: [], ReadSchema: struct<>

Problem is happening when HashAggregate(keys=[], functions=[], output=[]) node yields one row to partial_count node, which then captures one row. This four-node structure is constructed by AggUtils.planAggregateWithOneDistinct.

To fix the problem, we're adding Expand node which will force non-empty grouping expressions in HashAggregateExec nodes. This will in turn enable streaming zero rows to parent partial_count node, yielding correct final result.

Why are the changes needed?

Aggregation with DISTINCT literal gives wrong results. For example, when running on empty table t:
select count(distinct 1) from t returns 1, while the correct result should be 0.
For reference:
select count(1) from t returns 0, which is the correct and expected result.

Does this PR introduce any user-facing change?

Yes, this fixes a critical bug in Spark.

How was this patch tested?

New e2e SQL tests for aggregates with DISTINCT literals.

Was this patch authored or co-authored using generative AI tooling?

No.

[uros-db]

@cloud-fan @yaooqinn @viirya follow-up for SPARK-49000 is ready

back-ports are in separate PRs

[dongjoon-hyun]

In addition, please revise the PR title into a more corresponding one instead of sharing the same title with the original PR. The current PR has identical one, [SPARK-49000][SQL][FOLLOWUP] Fix "select count(distinct 1) from t" where t is empty table by expanding RewriteDistinctAggregates.

$ git log --oneline | grep SPARK-49000
dfa21332f20 [SPARK-49000][SQL] Fix "select count(distinct 1) from t" where t is empty table by expanding RewriteDistinctAggregates

[yaooqinn]

Thank you @uros-db @dongjoon-hyun @cloud-fan

Merged to master